The invention relates to valves and particularly to fluid actuated and retarded valve actuators. While the invention has particular application to butterfly valves it will be understood that it also has application to other types of valves including but not limited to poppet, globe and other rotary valves. Any valve that opens with difficulty is potentially an application for the present invention. Pneumatic valve actuators are widely used for operating valves and pneumatic valve actuators are particularly desirable because compressed air is often readily available, the systems are relatively insensitive to leakage in that there is relatively little impact on the control system if there is a minor leak and the leak does not cause contamination of the ambient.
Despite these advantages, any problems are peculiar to pneumatic valve actuators. More specifically, in relatively large valves utilizing a crank arm in combination with a pneumatic actuator, the valve butterfly may set very firmly and thus a very large force may be required to initially unseat the valve. Thereafter, only a relatively small force is necessary to move the valve butterfly further away from its seat. Thus, a pneumatic actuator that produces the required initial force will continue to produce that substantial initial force even though far less force is required to open the valve even a slight additional amount. In part this is a function of the substantial compressibility of the air that is used to open and modulate the valve.
It will be seen that pneumatic actuators will tend to overshoot. That is, the valve will tend to open much wider than is desired. The valve will tend to require a large force to unseat the butterfly which will cause the valve to open much wider than is desired and the control system will not reduce the pneumatic pressure applied to a pneumatic actuator once the valve butterfly has moved away from its seat.
The prior art includes the following U.S. Pat. Nos.: 621,388; 741,732; 756,291; 964,735; 1.274,195; 1,672,366; 1,366,081; and 4,471,937.
U.S. Pat. No. 1,274,195 discloses a valve in which a mechanical linkage is provided to move a piston to cause oil to flow into a chamber which can then be used to force a valve to open in the manner of conventionally hydraulically operated valve.
The prior art also includes hydraulic car lifts such as those that are typically used in service stations. The apparatus includes a hydraulic cylinder which is coupled to an oil reservoir. In a typical construction air is pumped into the top of the oil reservoir to force the oil in the hydraulic reservoir into the cylinder to move a ram or piston and thus raise the automobile.
It is an object of the invention to provide apparatus which may be retrofitted on existing valves as well as be installed on new valves at the time of original manufacture.
It is another object of the invention to provide apparatus which is relatively inexpensive to manufacture and simple to install.